Hot melt pressure sensitive adhesives (HMPSAs) have been used to bond a variety of materials to many substrates. A hot melt adhesive generally is a nearly water- and solvent-free adhesive which is applied on the substrate out of a molten state. The setting of the adhesive happens during the cooling down phase of the molten HMPSA, and retains high surface tackiness over time such that the HPMSA has a theoretically infinite open time. The bonding force of the HPMSA is derived from the ability of the adhesive to be compatible with the surface of both the substrate and the bonded materials. The adhesive bond arises from the compatibility of the adhesive mass with both the bonded material and the substrate and the internal cohesiveness of the adhesive mass.
Hot melt pressure-sensitive adhesives are widely used in the art. End uses include freezer tape, mailing tapes, peel and seal envelopes, outdoor labels, diapers, feminine sanitary napkins, adult incontinent products, medical gowns, and the like. The adhesive is applied to the article from a melt and are tacky at room temperature (or the temperature of desired use) for an extended period. These adhesives adhere essentially immediately to almost all substrates under light pressure. Although they can also be used for establishing bonds of large surface area after relatively long waiting times, the strengths of the resulting bonds are often very poor. Accordingly, they are mainly used when subsequent separation is required as is the case, for example, with adhesive plasters, labels, adhesive tapes, etc. To avoid unwanted bonding, the tacky surfaces have to be covered prior to use. One commonly used covering is silicone paper.
Hot melt pressure sensitive adhesives typically comprise a polymer, a tackifying resin and a plasticizing oil. The polymer provides flexibility, integrity and smooth peel adhesion properties. It also further provides a medium for dissolution or suspension of the tackifying resin and the plasticizing oil. The tackifying resin enhances tack properties and adhesion, and also reduces viscosity and the plasticizing oil reduces peel values, viscosities, glass transition temperatures and storage modulus (G′), and increases flexibility. The low ultimate peel value and lower peel adhesion of the removable adhesive is achieved through the use of higher polymer and plasticizing oil concentrations and lower tackifying resin concentrations in the adhesive. The low levels of tackifier and high polymer and oil concentrations can lead to several problems.
There are many requirements that must be met in the development of removable grade hot melt pressure sensitive adhesives including low viscosities at application temperatures of less than about 175° C., high heat resistance, good cold temperature flexibility, resistance to cold flow, good adhesion to the base substrate, high resistance to bleeding or staining of substrates, good die-cuttability and good quick tack or quick stick. The difficulty in formulating such adhesives is achieving a good balance of desirable properties without sacrificing one property to another. For instance, high heat resistance may be achieved at the expense of good cold temperature flexibility, lower viscosity may be achieved at the expense of stain resistance due to either higher levels of the liquid components or lower levels of polymer, good cold temperature flexibility may be achieved at the expense of resistance to what is known as “cold flow” at room temperature and adhesion and quick tack may be sacrificed to obtain lower, smoother peel characteristics to mention only a few examples.
Hot melt pressure sensitive adhesive formulations use a wide variety of thermoplastic elastomers (TPE's) including SIS, SBS, SEBS and SEPS where the S represents a styrene block, I is a polyisoprene block, B is a polybutadiene block, EB is an ethylene/butylene block and EP is a polyethylene/propylene block. Specific examples of hot melt pressure sensitive adhesives include ethylene-vinyl acetate (EVA) copolymer-based hot melts (U.S. Pat. No. 4,299,475), amorphous polypropylene-alpha-olefin (APAO)-based hot melts (U.S. Pat. No. 4,460,728), styrene-butadiene-styrene (SBS) A-B-A-B-A multi-block copolymer-based hot melts (U.S. Pat. No. 4,526,577), styrene-butadiene-styrene (SB)n radial copolymer-based hot melts (U.S. Pat. Nos. 4,944,933, 5,024,667, 5,037,411 and 5,057,571), and styrene-isoprene-styrene (SIS) A-B-A block copolymer-based hot melts (U.S. Pat. No. 5,149,741).
Although a broad state of the art for HMPSAs already exists, there is a need for such adhesives with improved properties. One area of need is for improved performance at low temperature. The present invention provides such an improvement.